Fracture fixation is a common surgical operation to address bone fractures by attaching a reinforcing rod or a plate or a cage to a fractured bone so that the broken ends may be stabilized to promote fusion and consequent healing.
Metal implants have often been used because of their high stiffness and strength that stabilizes the fracture site during tissue healing. However, several issues still remain. Metal implants, being much stiffer than bone, become the primary load-bearing member thereby protecting the bone from stress, thus resulting in stress shielding. It has been observed that moderate stress is beneficial to bone tissue growth. If the metal implant is not removed, the extended stress shielding will eventually weaken the bone, resulting in bone atrophy through decalcification or osteoporosis. Therefore, it is often necessary to perform a second surgical procedure to remove metal implants after the bone tissues have healed. This second surgical procedure can result in pain and discomfort.
The use of bioabsorbable materials for fracture fixation has the potential to eliminate the necessity of a second operation and help alleviate the negative effect of stress shielding. Specifically designed bioabsorbable materials can have mechanical properties that begin to approach those of bone in some applications (but certainly not all) and are strong enough to stabilize the fracture. As time progresses, the bioabsorbable material implant gradually decreases its stiffness and strength due to biodegradation. At the same period of time, the bone fracture heals. During the course of these two overlapping events, the mechanical stresses from daily activity and exercise are gradually transferred from the degradable bioabsorbable implant to the healing bone tissue.
Bioabsorbable polymeric materials have also been used in the form of pins, rods, anchors and staples for a variety of medical applications including orthopaedic devices. They are usually made by injection molding or extrusion. However, relatively low stiffness and strength of bioabsorbable devices compared with metallic implants have limited their use to low-load bearing applications or non-load bearing applications.
Considerable effort has been applied towards increasing the stiffness and strength of bioabsorbable materials. Several methods have been reported using various composite technologies in attempt to increase the strength and stiffness of bioabsorbable polymers by using different methods and types of reinforcements.
One method using composite technology has been to incorporate a strong and stiff non-absorbable inorganic structural fiber or partiles, made from carbon or glass, as reinforcing agents in a bioabsorbable polymeric matrix. The disadvantage of this system is that the non-absorbable fibers stay behind in the body tissue after the bioabsorbable polymer has been absorbed by the body and in the long run, may cause tissue reaction or other undesirable effects, such as unwanted migration.
Another composite preparation method has been to incorporate inorganic bioabsorbable glass or ceramic reinforcement such as fibers or particles. The lack of fiber-matrix interfacial bonding remains a major issue, and may cause poor load transfer mechanism between the fiber and the incompatible matrix. Poor interface problems are accentuated when implants are placed in human body and may cause the implant to fail prematurely.
Another composite preparation method has been to reinforce bioabsorbable polymers with different polymer fibers. The reinforcing polymer fibers, are usually stiffer and stronger (usually having higher glass transition and melting temperatures) than the matrix. In yet another method, highly drawn fibers of polylactide (PLA) or polyglycolide (PGA) were fused to develop bioabsorbable polymeric device with increased stiffness and strength.
Several patents describe bioabsorbable composites and the way of making them. U.S. Pat. No. 5,674,286 describes bioabsorbable composites for use in medical implants. The composite materials are composed of a hybrid yarn of intimately co-mingled reinforcing fibers of a crystalline polymer and matrix fibers of a polymer having a glass transition temperature lower than the melting point of the fiber crystalline polymer. The hybrid yarn is heated under pressure to a processing temperature between the glass transition temperature of the matrix fibers and the melting temperature of the crystalline polymer to form the continuous fiber reinforced composites.
U.S. Pat. No. 5,092,884 describes a composite structure having two or more biocompatible fiber polymers, in which at least, one of the reinforcing fibers is nonabsorbable. The fiber is woven into a mesh and then encapsulated with at least one biodegradable polymer.
U.S. Pat. No. 4,279,249 describes an osteosynthesis part made from a bioabsorbable composite made from a matrix of lactic acid homopolymer or a copolymer high in lactic acid unit with embedded discrete reinforcing elements made from glycolic acid or copolymers predominant in glycolic acid unit.
U.S. Pat. Nos. 5,578,046 and 5,626,611 describe bioabsorbable composite materials having a core portion formed from a first bioabsorbable material and the shell portion of the second bioabsorbable material joined to the core portion. The first bioabsorbable polymer has a higher rate of bioabsorption than the second bioabsorbable polymer. The composite filaments can also be made into woven and non-woven sheets.
U.S. Pat. Nos. 5,468,544 and 5,721,049 describes layered composite materials formed from bone bioactive glass or ceramic fibers and structural fibers and bioabsorbable polymers.
U.S. Pat. No. 5,728,753 describes composites suitable for use as prostheses for attachment to soft tissue, such as cartilage, tendons, skin, tympani membrane and gingiva, as well as cancellous or trabecular bone, based on combinations of polyolefinic binders with certain bioactive glass particles and fibers.
U.S. Pat. No. 5,338,772 describes an implant material, which is based on porous composite materials formed by calcium phosphate ceramic particles bridged by bioabsorbable polymers.
U.S. Pat. No. 4,604,097 describes spun or drawn glass fibers for use in the areas of medical devices as reinforcement for bioabsorbable polymeric orthopedic and dental implants.
In the formation of reinforced biocompatible organic composites, the matrix polymer is melted, forced generally under pressure to flow around the reinforcing fibers and cooled. The consolidation temperature of the matrix can be either below or above the Tg of the fiber material. Unfortunately, the melting temperature of the matrix may cause the biocompatible organic fibers to partially relax their molecular orientation, thereby losing their strength and stiffness, which consequently affect the properties of the composite.
Thus it is an object of the present invention to provide biocompatible organic polymeric composites with one or more improved properties and a method for producing these composites.